Monthly Archives: March 2013

Been busy coding away on the Gauge Pod software. It’s coming along nicely. I’ve taken a slightly different approach from the first version and the code is turning out to be a lot simpler. Still using layered architecture to separate Sensors, Gauges, Communication and UI but I changed how the layers interact with each other. The update performance has gone up dramatically. Using the Gauge Pod Sender in USB mode has also greatly reduced overhead and feeds data incredibly fast.

I’m currently in the testing phase of the software but looking good so far. The gauges are VERY stable compared to the last version. Will start testing the software on the car in the next few weeks as time permits. Can’t wait to try this in the car.

I’ve spent about 2 days on dealing with the ADC (Analog to Digital Converter) of the Arduino platform. Turns out there’s more problems to overcome when dealing with taking analog signal from a sensor and converting it to meaningful digital reading.

The issue is that even with a constant input voltage the ADC reading sampling error rate is larger than I’d like (~1.2%) with some random spikes in excess of 3%.

I built a test rig to troubleshoot the issue. Using constant input voltage and a potentiometer to simulate the analog input from a sensor.

Things I found:
– Arduino Atmel chip uses a single ADC and multiplexes the input pins.
– Switching read pin on the ADC causes noise in the system. Solution to read the first value and totally discard it. Then wait at least 10ms before sampling actual data.
– Sample accuracy is inversely proportional to the delay between samples. Shorter delay between reads, less drift. But over a static period of time, the error rate is the same.
– ADC is quite sensitive to electronic noise. Adjacent pins seem to affect reading.

This is probably as close as I can get to get an accurate reading that doesn’t jump around too much. Filters most noise while giving a decent sample rate. Currently reading all 4 inputs. Technically could reduce to 3 inputs since 4th won’t be used for a while. Will see how well Gauge Pod software deals with current feed rate.

I’ve started working on Gauge Pod 2.0 for the Challenger. The original version had some issues as described here.

The original sender was located under the dash. Currently utilizes three sensors (AFR, Boost and Fuel Pressure). The box was built to support more sensors but in the end I ended up using only those three. I used 3 pin molex connectors which turned out to the a bad idea as the connections weren’t very secure. Additionally I had to make some last minute modifications to feed a 5V signal to the sensors. In the end the box was way too bulky and not terribly efficient.

Due to the fact that the Arduino Diecimila uses pin connections (as it’s more for prototyping), the connections weren’t very good. While I had no problem with the connections themselves, I didn’t feel that this was sufficient for long term use. The next sender will have everything properly soldered.

The enclosure I used was also too large even for the size of the board. The new Arduino board (Teensy) is tiny compared to the original board. Teensy is actually more powerful than the Diecimila board, as it offers double the RAM (whooping 2K), double the Flash (32K) and a lot of PWM outputs and Analog inputs. Additionally, the Teensy board can act as native USB device, no need for serial port emulation. It’s also quite cheaper than the official Arduino.

Sensor voltage is supplied via an RC BEC board from Castle Creations. The BEC can be programmed for any output voltage. Originally the board was programmed for 5.1V from factory. The BEC is most likely type of MOSFET voltage regulator instead of the more traditional linear regulator. This does introduce a bit of noise into the system as the PWM system typically operates in the Khz range. Though, with bench testing I didn’t see any significant problems with this.

I didn’t actually realize right away that I had a programmer for the BEC. I picked one up last year for my Mamba Pro ESC. With just a few clicks of a button I was able to program it for 5V.

One of my theories for the flactuating voltage is that one or more sensors was putting too much load on the BEC. During testing I noticed that with higher amperages the BEC supplied voltage would drop. At 0.7A the voltage dropped to almost 4V. This was quite unusual since the BEC is rated at 10A. To mitigate this problem I’m using some inline, low ohm resistors to current limit the supply. This should prevent excessive load on the BEC.

For the input voltage and sensors I’m using 2 and 4 pin barrel connectors.

The Blue indicator blinks when new voltage data is sent. The Green indicator is on when the sensor supply voltage is above 4.5V. The Green indicator changes to Red if the voltage drops below 4.5V

I’m using a small perf board to solder all the components on. There’s not much to it. A few 1/4W resisitors for current limiting sensors, few resistors for the RGB LED and a trim pot for adjusting data rate.

The box all done. There’s a small hole on the side for the mini-USB connector that will connect via USB cable to the carputer. This will in turn be read by the Gauge Pod software, parsed and displayed as an analog gauge on the screen.

Last step was to solder up some of the female connectors that will be spliced into the Challenger itself. Just three sensor connections for now. I might add oil pressure sensor later down the line.

Thanks to the use of the reference voltage on each input any fluctuations of supply voltage and ADC error will be smoothed out. The Arduino sketch is very simple. The sender simply sends tab delimited data to the USB device in the format:

{INPUT}\t{REF_VOLT}\t{SENSE_VOLT}\r\n

e.g. IN1\t5.00\t2.00\r\n

This is repeated for each sensor input and finally sends the supply voltage: REF\t5.00\r\n